Substations for power distribution in high and medium voltage power networks include primary or field devices such as electrical cables, lines, bus bars, disconnectors, circuit breakers, power transformers and instrument transformers arranged in switchyards and/or bays. These primary devices are operated in an automated way via a Substation Automation (SA) system responsible for controlling, protecting and monitoring of substations. The SA system comprises microprocessor based, programmable secondary devices, so-called Intelligent Electronic Devices (IED), interconnected in a SA communication network or local area network (LAN) and interacting with the primary devices via a process interface. The IEDs are generally assigned to one of three hierarchical levels, namely the station level including e.g. a Human-Machine Interface (HMI) and a gateway to a Network Control Centre (NCC), the bay level with its bay units for protection and control, and the process level directly at the switchgear. A process interface between the process and the bay level generally comprises isolation amplifiers for galvanic isolation of analogue inputs or optical couplers for galvanic isolation of binary process inputs.
IEDs on the bay level, also termed bay units, in turn are connected to each other and to the IEDs on the station level via an optical inter-bay or station bus. IEDs for protection and control are responsible for the protection of primary devices and continuously evaluate currents, voltages, temperatures measured locally, and locally disconnect lines or other components when these measurements indicate an emergency, overload or fault condition. These multi-functional protection and control devices with associated measurement and communication units are referred to as numerical, digital or protective relays. The monitoring, protection or control functions performed comprise for instance disturbance recording, over-current protection, differential protection, phase comparison, distance protection, breaker failure protection or bus bar protection.
For functions protecting against a failure of a primary device, like breaker failure protection or bus bar protection, which necessarily operate beyond the scope of the failing primary device, so called protection zones have to be considered. Protection zones are electrically connected parts of the switchyard, which are delimited by open disconnectors and open or closed circuit breakers. Accordingly, the relation between protection zones and switches, i.e. the disconnectors and circuit breakers, or their mutual assignment, is dynamically determined from the switchyard topology at single line level and from the present state of all disconnectors. Conventionally, even with distributed bus bar protection systems, the determination of the protection zones is done in one central IED that obtains all switch positions from the distributed bay units, calculates the zones, and occasionally transmits appropriate trip commands to the bays. In other words, both the calculation of the protection zones and the safekeeping and evaluation of this information are left to one central IED.
The concept of protection zones has been used for some time for breaker failure protection and for bus bar protection. By way of example, if a bay circuit breaker which is tripped by e.g. a line protection function would not open because of an internal failure, a so-called breaker failure protection function is triggered, and a trip signal is propagated to circuit breakers in protection zones adjacent to, i.e. to the left and right of, the failed circuit breaker. In other words, the task of breaker failure protection is to detect that a breaker has failed to clear a fault, and to trip all the remaining breakers feeding into the section containing the fault in order to clear the fault for good. Likewise, the task of bus bar protection is to detect any fault on the bus bar, and to trip the breakers connected to the affected bus bar.